In MRI, magnetic materials such as gadolinium chelates and magnetic nanoparticles are often employed to enhance image contrast (see, e.g., Lu et al. (2006) Nanotechnology 17: 5812-5820, which is referred to herein as, “Lu, J., et al.”) The magnetic nanoparticles are passivated by biocompatible coatings such as dextrin, citrate, polystyrene/divinylbenzene and elemental gold. These coatings also detoxify the particles, resulting in enhanced lifetimes in vivo. Typical examples of magnetic nanoparticulate core-shell configurations include magnetite-dextrin, magnetite-silica (see, e.g., Lu et al. (2007) Nano Letts., 7: 149-154) and iron-gold (see, e.g., Cho et al. (2006) Nanotechnology 17: 640-644).
Laser-based photoacoustic tomography (PAT) (see, e.g., Wang et al. (2003) Nat. Biotechnol, 21: 803-806) is a hybrid imaging modality. It uses a pulsed laser source to illuminate a biological sample. Light absorption by the tissue results in a transient temperature rise on the order of approximately 10 mK. The rapid thermoelastic expansion excites ultrasonic waves that are measured using broadband ultrasonic transducers conformally arranged around the sample. Finally, a modified back-projection reconstruction algorithm is used to construct a map of the distribution of the optical energy deposition within the sample. The spatial resolution of PAT is not limited by optical diffusion, but instead by the bandwidth of the acoustic detectors. It has been shown that PAT can depict subsurface tissue structures and functional changes noninvasively with a resolution better than 100 μm. Like other optical modalities, PAT is highly sensitive in mapping and quantifying the dynamic distribution of optical contrast agents such as metallic nanocolloids and organic dyes.
Dual modality nanoparticles for MRI and another imaging modality have been developed by others (see, e.g., Mulder et al. (2005) FASEB J., 19: 2008-2010; and Prinzen et al. (2007) Nano Letters 7: 93-100) None of these dual modality nanoparticles address the dual modalities of MRI and PAT.